Hepatitis B virus (HBV) is an oncogenic virus. An estimated 400 million individuals worldwide are infected with HBV leading to chronic hepatitis, cirrhosis, and hepatocellular carcinoma (HCC). The early events of HBV infection are poorly defined due to the low signal intensity, transient and dynamic nature of the entry process, and lack of a systemic approach to identify the cellular components required for individual steps. Once characterized, these host factors may serve as critical targets for therapeutic intervention, thus reducing the rate of chronic hepatitis and ensuing liver cancer. Duck hepatitis B virus (DHBV), the related hepatotropic DNA virus may serve as a model system for the identification of viral receptor/co-factors as well as for the evaluation of novel molecular targets for intervention of hepadnavirus infection. We have previously identified and cloned two DHBV pre-S envelope-interacting proteins: p170 (carboxypeptidase D, DCPD) and p120 (glycine decarboxylase). The DCPD has been established as a DHBV docking receptor but fails to confer DHBV susceptibility of cell lines, suggesting that other cellular co-factors are necessary for establishing productive viral infection. The p120 is distributed only in DHBV infectible tissues and essential for a post-binding step of the DHBV life cycle. Optimal p120 binding in vitro requires truncation of the pre-S domain, which contains a putative cleavage site for PC7, a proprotein convertase. Our long-term goal is to understand virus-host interactions in the viral entry pathway and identify cellular targets for therapeutic intervention. The present application will initiate investigations on the mode of p120 action. Our specific aims are: (1). Investigate the molecular basis whereby p120 mediates productive DHBV infection; (2). Determine the contribution of proprotein convertases to DHBV infectivity; and (3). Evaluate p120 as well as DCPD as molecular targets for intervention of DHBV infection. We hope these studies will contribute to the understanding of the early events of hepadnavirus infection and may lead to the development of novel antiviral strategies for prevention of HBV induced liver cancer.